Abstract: The medial arterial calcifications (MAC) observed in chronic kidney disease (CKD) progress irreversibly despite current treatment, and contributes to the poor cardiovascular outcomes in these patients. In addition, the risk of bone fracture in CKD patients has remained unchanged for the last 20 years despite improvement in the fracture risk in other mineralization disorders. The lack of progress suggests that despite the significant medical need, current therapeutics treating the mineralization disorder present in CKD-MBD are ineffective. Pyrophosphate (PPi) is a direct inhibitor of hydroxyapatite formation, and patients on hemodialysis have reduced plasma PPi levels. The appearance and progression of MAC correlates inversely with plasma pyrophosphate (PPi) concentrations. In addition, patients with CKD also develop alterations in skeletal metabolism referred to as chronic kidney disease ? bone and mineral disorder (CKD-MBD). Reduced bone mineral density is also present in rodent models of deficient ENPP1 and low plasma PPi, supporting the notion ENPP1/ PPi may play a role in the bone mineralization disorder present in CKD-MBD. A major experimental barrier to investigating the role of ENPP1/PPi in CKD is the lack of an ENPP1 enzyme replacement therapy (ERT) which normalizes plasma [PPi] in vivo. We have developed a soluble ENPP1 biologic agent overcoming these barriers and have established its efficacy in a lethal genetic model of vascular calcification, low plasma PPi, elevated FGF23, and progressive uremia called Generalized Arterial Calcification of Infancy (GACI). The objective of this proposal is to determine the role of ENPP1/PPi in the mineralization imbalances present in CKD-MBD. Our central hypothesis is that correcting low plasma [PPi] and ENPP1 deficiency with ENPP1 ERT will inhibit medial wall vascular calcifications, improve arterial hemodynamic properties, and improve the bone mineralization deficits present in CKD-MBD. Our hypothesis is based on our preliminary data showing that murine models of CKD-MBD drop plasma [PPi] by 50%, and that treating these mice with ENPP1 ERT significantly reduces ectopic calcifications and improves bone biomechanics in these models. Our hypothesis is also supported by our own preliminary data that Enpp1 deficiency increases cortical bone mineralization which damages the cortical bone osteocyte microenvironment and significantly decreases bone formation rates, resulting in marked osteopenia. We will validate our hypothesis by demonstrating the efficacy of ENPP1 ERT in murine models of CKD-MBD by documenting the effects of the therapeutic on the increased ectopic soft tissue calcifications and decreased bone mineralization present in these models.